This application claims the benefit of Japanese Application No. 2002-048533 filed Feb. 25, 2002.
The present invention relates to an MRI (magnetic resonance imaging) apparatus and MRA (magnetic resonance angiography) imaging method, and more particularly to an MRI apparatus and MRA imaging method that can reduce degradation of image quality due to attenuation of signal intensity and can satisfactorily render blood flow even when fast blood flow and slow blood flow are simultaneously present in an imaged region.
A TOF (time-of-flight) technique is known as an example of conventional MRA imaging techniques for rendering blood flow.
In general, the TOF technique renders blood flow in white by utilizing an in-flow effect, by which NMR (nuclear magnetic resonance) signals from unsaturated blood flow that flows into a thick slab and that is not saturated by an RF (radio frequency) pulse is intensified as compared with NMR signals corresponding to surrounding tissue that is saturated by an RF pulse.
Examples of the MRA imaging according to the TOF technique will be outlined below.
FIG. 8 is a prior art explanatory diagram showing a relationship among an imaged region A, a slab Sxe2x80x2, and a flip angle xcex1 in imaging blood flow in the head H of a subject.
The thickness L of the imaged region A is 15 cm, for example.
The slab Sxe2x80x2 has a thickness equal to that of the imaged region A.
As indicated by a flip angle profile P61, the flip angle xcex1 has a constant value xcex1v with respect to the thickness direction Z of the slab Sxe2x80x2.
If fast blood flow is to be mainly imaged, the flip angle xcex1v is set to a small value (e.g., 20xc2x0); and if slow blood flow is to be mainly imaged, the flip angle xcex1 is set to a large value (e.g., 40xc2x0).
FIG. 9 is a prior art explantory diagram showing a flip angle profile P71 disclosed in Japanese Patent Application Laid Open No. H5-154132.
In the flip angle profile P71, the flip angle xcex1 varies with the position in the thickness direction Z. Specifically, the flip angle xcex1s is small near the neck in which blood flow is fast, and the flip angle xcex1e is large near the top of the head in which blood flow is slow, resulting in the linearly varying flip angle xcex1.
FIG. 10 is a prior art explanatory diagram showing yet another flip angle profile P81.
In the flip angle profile P81, the imaged region A is divided into a plurality of slabs Sa-Sf each having a thickness xcfx84 smaller than the thickness L of the imaged region A. The thickness xcfx84 is 2.5 cm, for example. The flip angle xcex1 has a constant value xcex1v with respect to the thickness direction Z.
In the conventional MRA imaging method described with reference to FIG. 8, the imaging time is reduced because the imaged region is defined as one slab Sxe2x80x2; however, since the entire imaged region A has a constant flip angle xcex1v, the method poses the problem that it is difficult to satisfactorily render the whole blood flow when fast blood flow and slow blood flow are simultaneously present in the imaged region A.
On the other hand, in the conventional MRA imaging method described with reference to FIG. 9, the imaging time is reduced because the imaged region is defined as one slab Sxe2x80x2; however, since the residence time of blood flow in the slab Sxe2x80x2 is long, the signal intensity in the distal portion attenuates, leading to a problem of degraded image quality.
Further, in the conventional MRA imaging method described with reference to FIG. 10, degradation of image quality due to attenuation of signal intensity is prevented because the imaged region is divided into a plurality of thin slabs Sa-Sf and the time during which blood flow resides in each slab Sa-Sf is reduced: however, since the entire imaged region A has a constant flip angle xcex1v, the method poses the problem that it is difficult to satisfactorily render the whole blood flow when fast blood flow and slow blood flow are simultaneously present in the imaged region A.
It is therefore an object of the present invention is to provide an MRI apparatus and MRA imaging method that can reduce degradation of image quality due to attenuation of signal intensity, and can satisfactorily render blood flow even when fast blood flow and slow blood flow are simultaneously present in an imaged region.
In accordance with its first aspect, the present invention provides an MRI apparatus characterized in comprising: static magnetic field generating means for generating a static magnetic field; gradient magnetic field generating means for generating a gradient magnetic field; RF pulse transmitting means for transmitting RF pulses with a flip angle profile whose flip angle varies with respect to the thickness direction in each of a plurality of adjacent slabs formed by dividing an imaged region and whose average flip angle differs for each slab; NMR signal receiving means for receiving NMR signals from a subject; and blood flow imaging means for conducting blood flow imaging based on said NMR signals.
In the MRI apparatus of the first aspect, the time during which blood flow resides in each slab can be reduced by dividing an imaged region into thin slabs, and thus, attenuation of signal intensity is reduced and image quality is improved. Moreover, since the flip angle is varied in each slab and further the average flip angle is differentiated for each slab, the slabs can be excited by respective flip angles fit to local variation of blood flow conditions; thus, the whole imaged region can be satisfactorily rendered even when fast blood flow and slow blood flow are simultaneously present in the imaged region.
In accordance with its second aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that the total number of slabs divided by said RF pulse transmitting means is in the range of 3 to 100.
In the MRI apparatus of the second aspect, since the lower limit of the total number of slabs is xe2x80x9c3xe2x80x9d, the blood flow residence time is reduced to ⅓ or less as compared with a case in which the imaged region is defined as one slab. Moreover, since the upper limit of the total number of slabs is xe2x80x9c100xe2x80x9d, the problem of an extremely lengthened total imaging time is avoided.
In accordance with its third aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that said RF pulse transmitting means transmits ramped RF pulses in which said flip angle linearly varies.
In the MRI apparatus of the third aspect, since the flip angle is linearly varied using ramped RF pulses, RF pulses can be generated and transmitted by relatively simple processing.
In accordance with its fourth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that said MRI apparatus comprises flip angle specifying means for specifying flip angles at both ends of said imaged region or at both ends of each said slab, and said RF pulse transmitting means transmits RF pulses whose flip angle linearly varies from the flip angle at one end to the flip angle at the other end.
In the MRI apparatus of the fourth aspect, by an operator etc. simply specifying flip angles at both ends of an imaged region, it is possible to define the property linearly varying from a flip angle at one end to a flip angle at the other end, and therefore, determination of the flip angles involves no cumbersome operation. Moreover, if the flip angles at both ends of each slab are specified, the flip angle property can be minutely defined for each slab.
In accordance with its fifth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that said MRI apparatus comprises flip angle specifying means for specifying flip angles at both ends of said imaged region or at both ends of each said slab, and said RF pulse transmitting means transmits RF pulses whose flip angle curvilinearly varies from the flip angle at one end to the flip angle at the other end.
In the MRI apparatus of the fifth aspect, by an operator etc. simply specifying flip angles at both ends of an imaged region, it is possible to define the property smoothly varying from a flip angle at one end to a flip angle at the other end. Moreover, if flip angles at both ends of each slab are specified, the flip angle property can be minutely defined for each slab.
In accordance with its sixth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that said MRI apparatus comprises flip angle and ratio factor specifying means for specifying a flip angle at one end of said imaged region and a ratio factor, and said RF pulse transmitting means transmits RF pulses whose flip angle linearly varies from said flip angle at one end to a flip angle at the other end obtained by multiplying said flip angle at one end by said ratio factor.
In the MRI apparatus of the sixth aspect, by an operator etc. simply specifying a flip angle at one end of an imaged region and a ratio factor, it is possible to define the flip angle linearly varying from one end to the other end of the imaged region.
In accordance with its seventh aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that said MRI apparatus comprises flip angle and ratio factor specifying means for specifying a flip angle at one end of each said slab and a ratio factor, and said RF pulse transmitting means transmits RF pulses whose flip angle linearly varies from said flip angle at one end to a flip angle at the other end obtained by multiplying said flip angle at one end by said ratio factor.
In the MRI apparatus of the seventh aspect, by an operator etc. simply specifying a flip angle at one end of each slab and a ratio factor, it is possible to define the flip angle linearly varying from one end to the other end of the slab.
In accordance with its eighth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that said RF pulse transmitting means transmits RF pulses having flip angles that match each other at an abutting portion between adjacent slabs.
In the MRI apparatus of the eight aspect, the flip angle is prevented from having discontinuity between the joint portion between slabs.
In accordance with its ninth aspect, the present invention provides an MRI apparatus characterized in comprising: static magnetic field generating means for generating a static magnetic field; gradient magnetic field generating means for generating a gradient magnetic field; RF pulse transmitting means for transmitting RF pulses with a flip angle profile whose flip angle differs for each of a plurality of adjacent slabs formed by dividing an imaged region; NMR signal receiving means for receiving NMR signals from a subject; and blood flow imaging means for conducting blood flow imaging based on said NMR signals.
In the MRI apparatus of the ninth aspect, the time during which blood flow resides in each slab is reduced by dividing an imaged region into thin slabs, and thus, image quality is improved. Moreover, since the flip angle is differentiated for each slab, the slabs can be excited by respective flip angles fit to local variation of blood flow conditions, and the whole imaged region can be satisfactorily rendered. Furthermore, if the same slab has a constant flip angle, processing can be simplified.
In accordance with its tenth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that said RF pulse transmitting means transmits RF pulses having a minimum value of said flip angle of 5xc2x0-30xc2x0 and a maximum value of said flip angle of 35xc2x0-90xc2x0.
In the MRI apparatus of the tenth aspect, since the minimum value of the flip angle is in the range of 5xc2x0 to 30xc2x0, the problem that too large a flip angle is specified for a blood flow region in which high blood flow rendering performance can be expected is avoided. Moreover, since the maximum value of the flip angle is in the range of 35xc2x0 to 90xc2x0, the problem that too small a flip angle is specified for a blood flow region in which the blood flow rendering performance tends to be poor is avoided.
In accordance with its eleventh aspect, the present invention provides an MRA imaging method characterized in comprising: dividing an imaged region into a plurality of adjacent slabs; transmitting RF pulses with a flip angle profile whose flip angle varying with respect to the thickness direction in each of said slabs and whose average flip angle differs for each slab, to collect NMR signals; and conducting blood flow imaging based on said NMR signals.
In the MRA imaging method of the eleventh aspect, the time during which blood flow resides in each slab can be reduced by dividing an imaged region into thin slabs, and thus, attenuation of signal intensity is reduced and image quality is improved. Moreover, since the flip angle is varied in each slab and further the average flip angle is differentiated for each slab, the slabs can be excited by respective flip angles fit to local variation of blood flow conditions; thus, the whole imaged region can be satisfactorily rendered even when fast blood flow and slow blood flow are simultaneously present in an imaged region.
In accordance with its twelfth aspect, the present invention provides the MRA imaging method having the aforementioned configuration, characterized in that the thickness of said slabs is in the range of 1.5 mm-5 cm.
In the MRA imaging method of the twelfth aspect, since the lower limit of the thickness of slabs is 1.5 mm, the problem of an extremely lengthened imaging time due to a very large total number of slabs is avoided. Moreover, since the upper limit of the thickness of slabs is 5 cm or less, the problem that the time during which blood flow resides within one slab lengthens and signal intensity from the blood flow attenuates is avoided.
In accordance with its thirteenth aspect, the present invention provides the MRA imaging method having the aforementioned configuration, characterized in that said RF pulses are ramped RF pulses in which said flip angle linearly varies.
In the MRA imaging method of the thirteenth aspect, since the flip angle is linearly varied using ramped RF pulses, RF pulses can be generated and transmitted by relatively simple processing.
In accordance with its fourteenth aspect, the present invention provides the MRA imaging method having the aforementioned configuration, characterized in that, when the head of a subject is defined as an imaged region and imaging is conducted by dividing the imaged region into a plurality of slabs adjacent to each other from the carotid artery to the top of the head, the average flip angle in slabs near the carotid artery is smaller than the average flip angle in slabs near the top of the head.
In the MRA imaging method of the fourteenth aspect, an area near the carotid artery in which blood flows fast is excited by a relatively small average flip angle. On the other hand, an area near the top of the head in which blood flows slowly is excited by a relatively large average flip angle. As a result, blood flow throughout the head can be satisfactorily rendered.
In accordance with its fifteenth aspect, the present invention provides an MRA imaging method characterized in comprising: dividing an imaged region into a plurality of adjacent slabs; transmitting RF pulses with a flip angle profile whose flip angle differs for each said slab, to collect NMR signals; and conducting blood flow imaging based on said NMR signals.
In the MRA imaging method of the fifteenth aspect, the time during which blood flow resides in each slab is reduced by dividing an imaged region into thin slabs, and thus, image quality is improved. Moreover, since the flip angle is differentiated for each slab, the slabs can be excited by respective flip angles fit to local variation of blood flow conditions, and the whole imaged region can be satisfactorily rendered. Furthermore, if the same slab has a constant flip angle, processing can be simplified.
In accordance with its sixteenth aspect, the present invention provides the MRA imaging method having the aforementioned configuration, characterized in that blood flow imaging utilizing a TOF effect is conducted.
In the MRA imaging method of the sixteenth aspect, a blood flow image of high quality can be produced by using a TOF effect (in general, an in-flow effect).
According to the MRI apparatus and MRA imaging method of the present invention, the time during which blood flow resides in each slab is reduced, attenuation of signal intensity is reduced, and image quality is improved, by reducing the thickness of the individual slabs. Moreover, rendering performance is improved overall for blood flow having different velocities by differentiating the flip angle for each slab corresponding to variation of blood flow conditions in an imaged region. Especially, local variation of blood flow velocity can be minutely accommodated by the flip angle varying within each slab, thus offering enhanced clinical usefulness.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.